Adapting Bimetallic Thermostat Draft Control to Warm Morning

I am a manufacturing engineer by trade and often have to stay late to sort problems out. As such, the missus often is stuck in charge of the stove until 7 or 8 at night. Our coal doesn't cooperate particularly well with just dumping in a hod, setting the air, and walking away, especially with 2 18 month old twins, a 10 year old boy, and an eleven year old boy.

To try to make things a bit simpler, I found a bimetallic thermostat element marked as being a replacement part for a king stove. I've bent up a sheetmetal enclosure with mounting tabs and bought some light chain. I intend to try to use the thermostatic draft control to rotate the draft control on the stove's ash pan door, though that may not work if the stock draft control is too heavy or won't easily move.

The part that hangs me up is figuring out where to mount the bimetallic element. If there is an air gap between the external, porcelain-coated skin and the metal of the combustion chamber, I could use short screws. I, however, have no clue how much of a gap there may or may not be. Magnets may work but may also lose strength and slide or drop the thermostat .

Anyone have any insight or alternate ideas?

voodoochylde wrote:I am a manufacturing engineer by trade and often have to stay late to sort problems out. As such, the missus often is stuck in charge of the stove until 7 or 8 at night. Our coal doesn't cooperate particularly well with just dumping in a hod, setting the air, and walking away, especially with 2 18 month old twins, a 10 year old boy, and an eleven year old boy.

To try to make things a bit simpler, I found a bimetallic thermostat element marked as being a replacement part for a king stove. I've bent up a sheetmetal enclosure with mounting tabs and bought some light chain. I intend to try to use the thermostatic draft control to rotate the draft control on the stove's ash pan door, though that may not work if the stock draft control is too heavy or won't easily move.

The part that hangs me up is figuring out where to mount the bimetallic element. If there is an air gap between the external, porcelain-coated skin and the metal of the combustion chamber, I could use short screws. I, however, have no clue how much of a gap there may or may not be. Magnets may work but may also lose strength and slide or drop the thermostat .

Anyone have any insight or alternate ideas?

the bimetal draft controls on stoves typically use their own draft flapper and do not adjust the primary rotary or sliding draft controls on the front bottom door (i don't think it would be logical to attempt doing such a thing without the assistance of a motor of some sort). even on a stove with these built in (generally at the rear of the stove for aesthetics & to keep them well away from any ash build up btw) one of the complaints I hear at times is either the flapper wont close all the way or the chain drive does not pull it open enough, etc. (mind you... this is a very light flap)... to think you could reliably count on a mechanism like this to move a primary draft control on a bottom door (which at times is in direct contact with ash, clean outs, opening and closing, etc.... I believe would be a waste of time).

i know its not what you wish to hear but its my two cents for the little its worth.

EDIT: I did a simple search of this fine forum and came up with this thread.... might give you some hope, but I still think my previous comments have valid points .... 818 Warm Morning

Actually, it is precisely the sort of feedback I'm after. I'm still in planning and would much rather separate what will work from what probably won't. I had my suspicions that the bimetallic spring wouldn't be able to generate nearly enough force to do the job.

Thank you for the reality check.

I you can find one that does work, you need to know where it will work. For that you need the heat range specs for the bi-metallic.

Depending on where you mount it and how hot it gets, over time, bi-metallic strips lose some of their spring temper and thus some of their range of motion.

Example. Some early carburetor automatic choke stoves were mounted down on the motor where they got really hot. They didn't last as long as the round automatic chokes that were up on the carburetors. But even those, after a few years, couldn't travel enough to close the choke on that first cold weather morning, so the car wouldn't start. If they were rotated to close in cold weather, then they couldn't open enough and the carb ran too rich. Only replacing it with new choke coil got the adjustment range back to normal.

You need to know what temp ranges the bi-metallic is designed to work with and then see if you can find a place on the stove that works.

Paul

I agree, Paul. Now that moving the stock draft control is out of the picture, I'll probably be making my own version of the Harman I saw here with similar mods. Using box tubing and light-gauge sheet steel will allow me to tune with chain length and intake opening diameters.

Didn't NOSMOKE install a bimet on his Pot Belly stove? Seems I remember something about that. I know he did a bunch of work on it. Might be worth searching member NOSMOKE posts as well.

I should add that whatever you design, design it so that if the bi-metallic ever fails, it fails safe.

In other words, gravity and chimney draft work together to shut the air damper so the stove doesn't burn out of control.

Paul

If you need a heater that does not need tending much check into the combustioneer stoves they are out of production but, are common and many replacement parts can be had for them. They work great unless the power goes out.

Right up your alley, how about a 24 volt actuator controlled w/ a t'stat?
I have done it for a long time in the past w/ great success.
It fails closed...with power loss. The bimetallic is not dependant on power, though.
If you want I can spec. the part.

McGiever wrote:Right up your alley, how about a 24 volt actuator controlled w/ a t'stat?
I have done it for a long time in the past w/ great success.
It fails closed...with power loss. The bimetallic is not dependant on power, though.
If you want I can spec. the part.

You hit the nail on the head. I'm looking for a non-electric solution. I enjoy the thought of all subsystems functioning even if the power goes out.

As for the suggestion to find a combustioneer, I'd like to BUT I'm hoping to find a hand-fired boiler for next season. I'd love to do underfloor radiant. I don't want to split my efforts or finances too much. The 818 is great though it may be getting swapped for a 460B I have that needs work.

So, anyone have an ingenious method for attaching the thermostat itself? The only other thought I had was strapping it to the stove...

Thanks for all the replies, fellers. As always, this board is a hugely helpful source of information.

Oh, almost forgot! Paul, I am researching thermal links that release around 800 degrees.

To make your bi-metallic coil actuate effectively to allow air entry into the ash pan door it may req. the typical light weight flap w/ hinge...don't think you will have enough force to rotate and besides, un-rotate sounds even more difficult. The coil, as you know, needs to modulate...open-close,open-close to hold steady temp...just saying hinge is better than rotary.

I'm with McGiever on the lightweight flap vs attempting to rotate using a bimetal thermostat. Here are a few pics of an adjustable one I used trouble free for over 30 years . Simple but reliable.

You may want to check out the fusible links used in the Ansul fire suppression systems. The only problem I see with the link idea is most of the time the links are on a tensioner and are slowly pulled apart.

Paul, that vertical flapper worked well for you? Most I've seen are on an angle and are offset a couple inches away from the firebox. I can easily seal that style flapper with silicone sheet from work. Maybe a magnet to reduce flutter and noise. I definitely like the simplicity and ease of construction.

I'm curious whether lead (melting between 600 and 700 degrees) might work for a safety. I could mold lead to be the first link coming from the spring.

With such a lightweight door, I'd think you wouldn't have to worry about lead strain creep.

Depending on where you have to put it, you could alloy it to let go at lower, or higher temps. Even if you have to put the link in an area that is insulated, Ferro metal melts in the low 100 F range. If you've never worked with it, it's neat stuff to do low-temp metal castings with just using boiling water to melt it.

Paul

Orrrr, you could order the whole bi-metallic assembly from HITZER---it's been done & worked well on different stoves